This invention relates to hydraulic torque wrenches used for rotating mechanical components, for example for tightening or loosening nuts, bolts and screws.
Hydraulic torque wrenches commonly comprise a hydraulic piston-cylinder assembly the free end of the piston of which is pivotally connected to a drive lever or levers rotatable by the piston-cylinder assembly about an axis radially spaced from the pivot connection. The drive lever or levers carries holding means for the component to be turned, said holding means being rotatable co-axially with the drive lever or levers and being connected to said drive lever or levers by a ratchet. The holding means may comprise a shaft carrying a removable socket or a socket within the drive lever or levers.
In most known arrangements, the drive lever oscillates in an arc about the axis of the holding means and the distance between the line of action of the piston rod and said axis varies throughout the oscillation. In theory, when a constant force is applied, the torque exerted on the holding means is proportional to this distance.
In order to minimize inaccuracies, the stroke of the piston is kept quite short about top-dead-center, typically such as to rotate the holding means through an angle of the order of 10.degree. and whereby the arcing of the pivot point of the drive lever with the piston can be considered as substantially linear.
However it is an inherent feature of such designs that the distance between the line of action of the applied force and the axis of the holding means does vary slightly during the piston stroke, and the torque values calculated for a given hydraulic pressure can only be guaranteed within certain limits.
Further, the requirement to utilise a short piston stroke in an attempt to maintain accuracy means that a tightening operation can be a time consuming exercise.